Newsroom Midwest Region

Seeing Life in the Undead

Understanding the Importance of Snags in Forest Ecology

October 27, 2015

Pileated woodpecker activity and pileated woodpecker. Photo by Courtney Celley and Jim Hudgins/USFWS.
Pileated woodpecker activity and pileated woodpecker. Photo by Courtney Celley and Jim Hudgins/USFWS.

Our forests are full of life, even when they are dead. It may seem kind of gross to think about death and decay on a beautiful fall day, but with Halloween on the wind and kids donning their spooky costumes, it’s the perfect time to talk about one of nature's best kept secrets of the undead world...snags.

Within forested ecosystems, one dying tree can translate to new life and new opportunities for a host of living things. For a forest plant, for instance, a dying tree releases all sorts of resources and makes room for new and different plants to grow. More light may now filter down through the canopy, which spurs germination and other processes. As the tree decomposes, the nutrients once stored within the tree are released and made available to plants that were once its competitors.

For wildlife, these standing dead trees, commonly known as snags, now present a unique structure and potential food source. In turn, ants, beetles, and other wood-boring invertebrates that excavate and live within these snags, become a meal for many birds. Cavity nesters, such as the pileated woodpecker, might view snags not only as a food source, but also as a place to take up residence and nest.

Snag research study at Seney National Wildlife Refuge by Tina Shaw/USFWS.
Snag research study at Seney National Wildlife Refuge by Tina Shaw/USFWS.

The life of a snag only grows more extensive as it ages. Once cavity nesters like woodpeckers are finished nesting in snags, the cavities can be utilized by all sorts other wildlife, such as red squirrels, fishers, and many species of birds and bats. While the positive attributes of standing dead trees might seem like a happy accident for wildlife, some birds such as eastern bluebirds and wood ducks depend on them. They did not evolve with artificial nesting boxes on the landscape, they evolved with dead and dying trees.

“At the time a tree dies, it has only partially fulfilled its potential ecological function,” writes Dr. Jerry Franklin, a leading forest ecologist from the University of Washington.

Major factors are associated with the global biodiversity crisis including land use change, human population growth, and ecosystem simplification. To address the issue of ecosystem simplification, scientists like Dr. Franklin have devoted their careers to understanding forest dynamics, including patterns of tree death and decay. Biological legacies, such as snags, are remnants of forests left behind after major disturbance events. Based upon his pioneering work following the 1980 eruption of Mount Saint Helens, Dr. Franklin has promoted the understanding of patterns brought about by such events and using this in management of forests.

As Dr. Franklin noted, there are many factors and interactions that lead up to the point when a tree is considered “dead” through a combination of biotic and abiotic processes. Understanding these complex interactions is just one of the challenges land managers face, while balancing ecosystem function with other biological objectives, such as managing for specific species. In some forest ecosystems, biodiversity and biomass issues may not be solved by nest boxes that aim to mimic the natural functions of snags.

You’ve seen one tree, you’ve seen them all. Not so!

While scientists and land managers know the importance of snags, there is still a lot to be understood about managing for dead material among different ecosystems. The ecological significance of these standing dead trees can vary depending on the species of tree and local environmental conditions that helped the tree grow and live over time. The agents at play in the death of the tree also teach us lessons. Insects, disease, wind, and fire, can each influence tree death, decomposition, and wildlife use in different ways.

For land managers, understanding the complexities of snag management is important. Many wildlife species are dependent on structures such as snags, and a manager can place snags on a landscape through a variety of means, and in a variety of spatial arrangements, however, understanding the results of such treatments is key, particularly in Great Lakes region, where snag dynamics are relatively understudied.

We need to know more

At Seney National Wildlife Refuge, Master's candidate Shelby Weiss from Ohio State University, Dr. Greg Corace, of Seney National Wildlife Refuge, and Dr. Eric Toman, of Ohio State University, aim to answer questions about snags - about how they form and decay under different mortality agents and how they are being used by insects, woodpeckers and other wildlife.

From past published research, we know that snags of different tree species decay at different rates, and the rate at which snags decay also depends on the means by which they were created. With this information as a backdrop, we can now explore what this means to wildlife - will snag use also be influenced by how it was created?

Learn more: